This is a job for an outer join:
FROM tblClient T1
LEFT OUTER JOIN tblPortfolio T2 ON T1.clientID=T2.clientID
WHERE T2.clientID IS NULL;
The idea is that an OUTER JOIN like this will pair every row in the first table with matching entries in the second, based on the ON condition. There will be a row for every entry in the first table and if there is no matching entry in the second, then the columns of the second table will have their values set to NULL. The WHERE clause selects those nonmatching clients - that is, the second table's clientID is null.
Custom Apps, Inc.